Transmitters form a significant portion of most communication circuits. As such, they assume a position of prominence in design concerns. With the proliferation of mobile terminals, transmitter design has progressed in leaps and bounds as designers try to minimize components and reduce size, power consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers within limited bandwidth constraints.
One relatively recent change has been the advent of the Enhanced Data for GSM Evolution (EDGE) scheme in GSM systems. This system contains amplitude modulation components, and, as a result, the power amplifier must be linear and should not operate in saturation when classical modulation techniques are employed. Such a linear system lacks the efficiency of one that operates the power amplifier in saturation.
If a polar modulation system is used instead of a classical modulation system, then the power amplifier may operate in saturation and efficiency is greatly improved. In a polar modulation system, the amplitude component of a polar signal controls the power amplifier and will cause unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called amplitude modulation to phase modulation (AM to PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude. Further, the AM to PM distortion of the power amplifier may vary depending on factors such as the radio frequency (RF) power at the input of the amplifier and temperature. Thus, there remains a need for a system that compensates for variations in the AM to PM distortion of the power amplifier.